A partial melting study of an ordinary (H) chondrite composition with application to the unique achondrite Graves Nunataks 06128 and 06129

Abstract

Melting experiments of a synthesized, alkali‐bearing, H‐chondrite composition were conducted at ambient pressure with three distinct oxygen fugacity conditions (IW‐1, IW, and IW+2). Oxygen fugacity conditions significantly influence the compositions of partial melts. Partial melts at IW‐1 are distinctly enriched in SiO2 relative to those of IW and IW+2 melts. The silica‐enriched, reduced (IW‐1) melts are characterized by high alkali contents and have silica‐oversaturated compositions. In contrast, the silica‐depleted, oxidized (≥IW) melts, which are also enriched in alkali contents, have distinctly silica‐undersaturated compositions. These experimental results suggest that alkali‐rich, felsic, asteroidal crusts as represented by paired achondrites Graves Nunataks 06128 and 06129 should originate from a low‐degree, relatively reduced partial melt from a parent body having near‐chondritic compositions. Based on recent chronological constraints and numerical considerations as well as our experimental results, we propose that such felsic magmatism should have occurred in a parent body that is smaller in size and commenced accreting later than those highly differentiated asteroids having basaltic crusts and metallic cores.

To be or not to be oxidized: A case study of olivine behavior in the fusion crust of ureilite A 09368 and H chondrites A 09004 and A 09502

“…The composition of the secondary olivine might be also controlled by the oxygen fugacity ( f O 2 ) in the melt, as constrained in melting experiments for ordinary chondrite (Usui et al. ). However, effects induced by variation of f O 2 are relatively negligible for the Fe 3+ /Fe 2+ ratio in olivine (e.g., Putirka ).…”

Petrology of a nonindigenous microgranitic clast in polymict ureilite EET 87720: Evidence for formation of evolved melt on an unknown parent body

“…Usui et al. () studied low pressure experimental partial melting of a H chondrite analog which showed that siliceous (quartz‐normative) melts can be formed by very small degrees of melting under reducing conditions. Experimental studies by Gardner‐Vandy et al.…”

Normal to inverse transition in martian spinel: Understanding the interplay between chromium, vanadium, and iron valence state partitioning through a crystal-chemical lens

“…Other achondrites are derived from parent bodies with f O2 that are not well defined (e.g., brachinites, unclassified Na-rich achondrites such as GRA 06129/06128). As has been demonstrated, differences in the f O2 (IW-1 to IW+2) during partial melting on these parent bodies will produce considerably different melt compositions (e.g., Jurewicz et al 1995;Gardner-Vandy et al 2013;Usui et al 2015). Several studies have linked brachinites to unique sodium feldspar-rich achondrite GRA 06129/06128 (e.g., Shearer et al 2010;Usui et al 2015).…”

“…As has been demonstrated, differences in the f O2 (IW-1 to IW+2) during partial melting on these parent bodies will produce considerably different melt compositions (e.g., Jurewicz et al 1995;Gardner-Vandy et al 2013;Usui et al 2015). Several studies have linked brachinites to unique sodium feldspar-rich achondrite GRA 06129/06128 (e.g., Shearer et al 2010;Usui et al 2015). Usui et al (2015) illustrated that partial melting of a chondritic precursor would yield a brachinite residuum and a GRA 06129 melt, under specific f O2 conditions.…”

“…Several studies have linked brachinites to unique sodium feldspar-rich achondrite GRA 06129/06128 (e.g., Shearer et al 2010;Usui et al 2015). Usui et al (2015) illustrated that partial melting of a chondritic precursor would yield a brachinite residuum and a GRA 06129 melt, under specific f O2 conditions. As both lithologies contain Cr-spinels, examining the systematic relationship between their chemistries and f O2 would allow for an analysis of conditions of melting and crystallization.…”